Composite materials comprising a reinforcing material and a star polyamide as a thermoplastic matrix, the precursor compound article of said materials and the products obtained using same

ABSTRACT

The invention relates to a precursor article of a composite material comprising a polymer matrix and at least one reinforcing wire and/or fibres, said article comprising at least one reinforcing wire and/or fibres and at least one polymer matrix wire and/or fibres. The invention is characterised in that: said reinforcing wire and/or fibres are made from a reinforcing material and may comprise a thermoplastic polyamide part; said polymer matrix wire and/or fibres are made from thermoplastic polyamide; and the thermoplastic polyamide of said reinforcing wire and/or fibres and/or said polymer matrix wire and/or fibres comprise at least one polyamide with a star structure which contains: star macromolecular chains comprising one or more cores and at least three branches or three polyamide segments which are linked to the core; if necessary, linear polyamide macromolecular chains.

This application is an application under 35 U.S.C. Section 371 ofInternational Application Number PCT/FR02/03326 filed on Sep. 30, 2002.

The field of the invention is that of composite materials and of theprocesses for their manufacture.

More specifically, the invention relates to the use of a polyamidepossessing a star structure which is employed for the impregnation ofreinforcing materials, particularly in the form of yarns and/or offibers, which are intended to act as thermoplastic matrix, in compositematerials.

The term “yarn” is understood to mean a monofilament, a continuousmultifilament yarn or a strand of fibers obtained from a single type offiber or from several types of fibers in an intimate mixture. Thecontinuous yarn can also be obtained by combining several multifilamentyarns.

The term “fiber” is understood to mean a filament or a combination ofcut, split or converted filaments.

In the field of high performance materials, composites have assumed adominating position because of their performance and the savings inweight which they allow; The currently most well known high performancecomposites are obtained from thermosetting resins, the use of which islimited to small-scale applications, mainly in aeronautics or motorsports, and, in the best cases, which exhibit manufacturing times in theregion of approximately fifteen minutes, such as, for example, duringthe manufacture of skis. The cost of these materials and/or themanufacturing times render them incompatible with use in massproduction.

One reply, in regard to the manufacturing times, is given by compositescomprising a thermoplastic matrix. Thermoplastic resins are generallyknown for their high viscosity, which constitutes a check as regards theimpregnation of the reinforced materials, generally composed of verydense multifilament bundles. The use of the thermoplastic matricesavailable on the market, in particular polyamide matrices, results in adifficulty in impregnation, requiring either prolonged impregnationtimes or significant processing pressures. In most cases, the compositematerials obtained from these matrices may exhibit microspaces andunimpregnated regions. These microspaces bring about declines inmechanical properties, premature aging of the material and problems ofdelamination when the material is composed of several reinforcinglayers.

Several routes have been explored to improve the impregnation of thereinforcing yarns by the matrix and the adhesion between the reinforcingyarns and the matrix.

The first of these routes has consisted in using linear polyamides witha reduced molecular weight as matrix.

Thus, the document FR-2 158 422 discloses a composite sheet composed ofa polyamide matrix and of reinforcing fibers of glass fiber type. Thepolyamide is obtained by polycondensation of ε-caprolactam, themolecular weight of which is between 3000 and 25 000, having theability, by virtue of its low viscosity and therefore its low surfacetension, to suitably impregnate the reinforcing fibers and thus to limitthe appearance of microspaces in the finished product. This documentalso discloses a process for forming this composite sheet.

Generally, the use of polyamides of low molecular weights in the matrixexhibits the major disadvantage of detrimentally affecting themechanical properties of the composite, in particular as regards theultimate strength, the yield strength and the fatigue behavior. This isbecause, during the use of high performance composites reinforced bylong fibers, the mechanical properties of these composites depend on theplasticity of the matrix, which transmits the stresses to thereinforcing material, and on the mechanical properties of the matrix.

Another route which makes it possible to improve the impregnation of thereinforcing fibers by the matrix consists in employing a matrix which isprovided in the form of an oligomer or of a prepolymer of low molecularweight which can be polymerized by polycondensation in situ.

Thus, the document FR-A-2 603 891 relates to a process for themanufacture of a composite material composed of a polyamide matrixreinforced by long reinforcing fibers. These fibers are impregnated witha polyamide prepolymer or oligomer which comprises, at each end of themolecular chain, a reactive functional group capable of reacting withanother oligomer or prepolymer molecule under the effect of heating,resulting in the elongation of the polymer chain to produce a polyamideof high molecular weight. The oligomer or prepolymer of low molecularmass has the characteristic of being fluid in the molten state. Thepolyamides used are preferably polyamide-6, polyamide-6,6,polyamide-6,10, polyamide-6,12, polyamide-11 and polyamide-12. Theimpregnated fibers are subsequently pultruded through a shaping die athigh temperature in order to form profiles.

This process remains similar to conventional polymerization processesand thus exhibits cycle times incompatible with a rapid production rate.If the cycle times are adjusted so as to render them compatible withmass production, the molecular weight of the polyamide obtained andconstituting the matrix is too low to confer, on the matrix, asatisfactory level of mechanical properties.

The document EP-B-0 133 825 discloses a flexible composite materialmainly composed of a reinforced material in the form of a lock ofparallel continuous fibers which are impregnated with thermoplasticpowder, preferably with polyamide powder, and of a thermoplastic matrixin the form of a sheath around the lock of continuous fibers, it beingpossible for this sheath also to be made of polyamide. This material ischaracterized in that the polymer constituting the thermoplastic matrixhas a melting point lower than or equal to that of the polymerconstituting the thermoplastic powder, so that the sheathing of thefibers covered with powder is achieved by melting the thermoplasticmatrix, but without melting the powder, so that the latter isolates thefibers from the sheath.

A disadvantage of the use of a thermoplastic polymer in the form of apowder is the need to use complex equipment which limits the amount ofcomposite obtained. It is therefore clearly apparent that this processis not very compatible with mass production.

The document U.S. Pat. No. 5,464,684 discloses a hybrid yarn comprisinga core of an intimate mixture of reinforcing filaments and of filamentsof polyamides of low viscosity, forming the matrix. This nucleus iscovered with a continuous yarn of polyamide, preferably of the same typeas that used for the nucleus. The polyamide used is of the nylon-6 ornylon-6,6 type but can also be composed of nylon-6,6T, nylon-6,10,nylon-10 or a polyamide of adipic acid and of 1,3-xylylenediamine. Thereinforcing fibers are carbon fibers or glass fibers.

The technique used to manufacture such a hybrid yarn is certainlysuitable for small-scale applications, such as the manufacture of tennisracquets. However, it is difficult to conceive of the use of such amethod on a larger scale.

The analysis of the state of the art shows that the improvement in theperformance of composite materials, centered on the improvement in theimpregnation of the matrix into the reinforcing material, does not meetthe requirements either of mechanical properties or of processing timeof the mass production applications targeted by the thermoplasticcomposite materials.

The object of the present invention is thus to overcome thesedisadvantages by providing a precursor article of a composite materialcomprising different types of yarns and/or of fibers and in particularat least one reinforcing yarn and/or reinforcing fibers and at least oneyarn and/or fibers which generate(s) a thermoplastic matrix exhibiting ahigh fluidity in the molten state, making possible very goodimpregnation of the reinforcing yarns and/or reinforcing fibers duringthe formation of the composite material. Such an article makes itpossible to obtain a composite material by a simple and rapidthermoforming technique.

Another object of the invention is to provide a composite materialobtained from this article and exhibiting good mechanical properties.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graphic illustrating the influence of temperature onflexural modulus of a material form according to the present invention.

Finally, a last object of the invention is to provide a compositematerial exhibiting an advantage in reducing manufacturing costs by theuse of machinery employing low pressures and shortened cycle times.

The invention relates to a precursor article of a composite materialcomprising a polymeric matrix and at least one reinforcing yarn and/orreinforcing fibers, said article comprising at least one reinforcingyarn and/or reinforcing fibers and at least one polymeric matrix yarnand/or polymeric matrix fibers, characterized in that:

-   -   said reinforcing yarn and/or said reinforcing fibers are made of        reinforcing material and optionally comprise a part made of        thermoplastic polyamide,    -   said polymeric matrix yarn and/or said polymeric matrix fibers        are made of thermoplastic polyamide, and in that    -   said thermoplastic polyamide of said reinforcing yarn and/or of        said reinforcing fibers and/or of said polymeric matrix yarn        and/or of said polymeric matrix fibers comprises at least one        polyamide possessing a star structure comprising:        -   star macromolecular chains comprising one or more cores and            at least three polyamide branches or three polyamide            segments bonded to a core,        -   if appropriate, linear polyamide macromolecular chains.

The polyamide possessing a star structure is a polymer comprising starmacromolecular chains and, if appropriate, linear macromolecular chains.The polymers comprising such star macromolecular chains are, forexample, disclosed in the documents FR 2 743 077, FR 2 779 730, EP 0 682057 and EP 0 832 149. These compounds are known to exhibit an improvedfluidity with respect to linear polyamides.

The star macromolecular chains comprise a core and at least threepolyamide branches. The branches are bonded to the core by a covalentbond, via an amide group or a group of another nature. The core is anorganic or organometallic chemical compound, preferably ahydrocarbonaceous compound optionally comprising heteroatoms and towhich the branches are connected. The branches are polyamide chains. Thepolyamide chains constituting the branches are preferably of the type ofthose obtained by polymerization of lactams or amino acids, for exampleof polyamide-6 type.

The polyamide possessing a star structure according to the inventionoptionally comprises, in addition to the star chains, linear polyamidechains. In this case, the ratio by weight of the amount of star chainsto the sum of the amounts of star chains and of linear chains is between0.1 and 1, limits included. It is preferably between 0.5 and 1.

According to a preferred embodiment of the invention, the polyamidepossessing a star structure, that is to say comprising starmacromolecular chains, is obtained by copolymerization of a mixture ofmonomers comprising at least:

-   a) monomers of following general formula (I):

-   b) monomers of following general formulae (IIa) and (IIb):

-   c) optionally monomers of following general formula (III):    Z-R₃-Z  (III)-    in which:    -   R₁ is an aliphatic or aromatic, cyclic or linear,        hydrocarbonaceous radical comprising at least 2 carbon atoms        which can comprise heteroatoms,    -   A is a covalent, bond or an aliphatic hydrocarbonaceous radical        which can comprise heteroatoms and which comprises from 1 to 20        carbon atoms,    -   Z represents a primary amine functional group or a carboxylic        acid functional group,    -   Y is a primary amine functional group when X represents a        carboxylic acid functional group or    -   Y is a carboxylic acid functional group when X represents a        primary amine functional group,    -   R₂ and R₃, which are identical or different, represent        substituted or unsubstituted, aromatic, cycloaliphatic or        aliphatic hydrocarbonaceous radicals comprising from 2 to 20        carbon atoms which can comprise heteroatoms,    -   m represents an integer between 3 and 8.

The term “carboxylic acid” is understood to mean carboxylic acids andtheir derivatives, such as acid anhydrides, acid chlorides, esters, andthe like.

Processes for producing these star polyamides are disclosed in thedocuments FR 2 743 077 and FR 2 779 730. These processes result in theformation of star macromolecular chains, as a mixture with optionallylinear macromolecular chains.

If a comonomer of formula (III) is used, the polymerization(polycondensation) reaction is advantageously carried out untilthermodynamic equilibrium is reached.

The monomer of formula (I) can also be blended with a molten polymerduring an extrusion operation.

Thus, according to another embodiment of the invention, the polyamidepossessing a star structure is obtained by melt blending, for exampleusing an extrusion device, a polyamide of the type of those obtained bypolymerization of lactams and/or amino acids and a monomer of formula(I).

Such preparation processes are disclosed in patents EP 0 682 070 and EP0 672 703.

According to a specific characteristic of the invention, the R₁ radicalis either a cycloaliphatic radical, such as the tetravalentcyclohexanonyl radical, or a 1,1,1-propanetriyl or 1,2,3-propanetriylradical.

Mention may be made, as other R₁ radicals suitable for the invention, byway of example, of substituted or unsubstituted trivalent phenyl andcyclohexanyl radicals, tetravalent diaminopolymethylene radicals with anumber of methylene groups advantageously of between 2 and 12, such asthe radical originating from EDTA (ethylenediaminetetraacetic acid),octavalent cyclohexanonyl or cyclohexadinonyl radicals, and the radicalsoriginating from compounds resulting from the reaction of polyols, suchas glycol, pentaerythritol, sorbitol or mannitol, with acrylonitrile.

Advantageously, at least two different R₂ radicals can be employed inthe monomers of formula (II).

The A radical is preferably a methylene or polymethylene radical, suchas the ethyl, propyl or butyl radicals, or a polyoxyalkylene radical,such as the polyoxyethylene radical.

According to a specific embodiment of the invention, the number m isgreater than or equal to 3 and advantageously equal to 3 or 4.

The reactive functional group of the multifunctional compoundrepresented by the symbol Z is a functional group capable of forming anamide functional group.

Preferably, the compound of formula (I) are chosen from2,2,6,6-tetra(β-carboxyethyl)cyclohexanone, trimesic acid,2,4,6-tri(aminocaproic acid)-1,3,5-triazine and4-aminoethyl-1,8-octanediamine.

The mixture of monomers which is the source of the star macromolecularchains can comprise other compounds, such as chain-limiting agents,catalysts or additives, such as light stabilizers or heat stabilizers.

The polyamide yarn and/or polyamide fibers intended to act as matrixwill be referred to hereinafter as “matrix yarn and/or matrix fibers”.

Advantageously, the polyamide possessing a star structure exhibits anumber-average molecular mass at least equal to 15 000.

Advantageously, when the reinforcing yarn and/or reinforcing fiberscomprise a polyamide possessing a star structure, the latter ispreferably provided in the form of a sheath of polyamide which coversthe reinforcing yarn and/or reinforcing fibers.

According to an alternative form of the invention, the matrix yarnand/or matrix fibers are obtained from a blend of the polyamidepossessing a star structure and of a linear polyamide.

According to another alternative form, the precursor article of thecomposite material also comprises at least one matrix yarn and/or matrixfibers made of linear polyamide.

According to a preferred characteristic, this linear polyamide is analiphatic and/or semicrystalline polyamide or copolyamide chosen fromthe group consisting of PA-4,6, PA-6, PA-6,6, PA-6,9, PA-6,10, PA-6,12,PA-6,36, PA-11 and PA-12, or a semicrystalline semiaromatic polyamide orcopolyamide chosen from the group consisting of polyphthalamides, andthe blends of these polymers and of their copolymers.

It is then advantageous for the ratio by weight of polyamide possessinga star structure in the matrix yarn and/or matrix fibers to be between0.4 and 1 and preferably at least equal to 0.6.

The matrix yarn and/or matrix fibers can also comprise all theconventional additives, such as flame-retardants, plasticizers, heat andlight stabilizers, waxes, pigments, nucleating agents, antioxidants,impact-strength modifiers or analogous compounds which are known to aperson skilled in the art.

Advantageously, the reinforcing yarn and/or reinforcing fibers arechosen from carbon, glass, aramid and polyimide yarns and/or fibers.

According to an alternative form of this characteristic, the reinforcingyarn and/or reinforcing fibers are a natural yarn and/or natural fiberschosen from sisal, hemp or flax yarns and/or fibers.

Advantageously, the article according to the invention also comprises apowdered material, the matrix precursor, which can, for example, be apolyamide.

Use will preferably be made of a powder exhibiting a particle size ofbetween 1 and 100 microns.

Preferably, the article according to the invention is the form ofcontinuous or cut yarns, slivers, mats, braids, woven fabrics, knittedfabrics, sheets, multiaxials, nonwovens and/or complex forms comprisingseveral of the abovementioned forms. By way of examples, a complex formcan be a sheet combined with a nonwoven or with continuous yarns.

Another subject matter of the invention is a composite material obtainedfrom an article as defined above by at least partial melting of thematrix yarn and/or matrix fibers. This composite material comprises apolymeric matrix and reinforcing yarns and/or reinforcing fibers.

The term “partial melting” is understood to mean the melting of at leasta part of at least one matrix yarn and/or one matrix fiber.

This melting can be carried out by thermoforming at a temperature moreor less equal to the melting point of the polymeric matrix and underpressure. This melting makes it possible to obtain homogeneousimpregnation of the reinforcing yarns and/or reinforcing fibers by thematrix.

According to a preferred characteristic, the composite material thusobtained exhibits a level of reinforcing material by weight of between25 and 80%.

Yet another subject matter of the invention is a semi-finished productobtained by a process of thermoforming or of calendering theabovementioned article, during which the matrix yarn and/or matrixfibers is/are at least partially melted in order to impregnate thereinforcing yarn and/or reinforcing fibers.

More advantageously, this semi-finished product is provided in the formof panels or of tapes.

The semi-finished product consists of an intermediate product, in whichthe reinforcing yarns and/or reinforcing fibers have been impregnated bythe polymeric matrix, which is found in the form of a continuous phase.This product is not yet in its definitive form.

The semi-finished product has to be subjected to a final stage offorming by a forming or thermoforming process which are known to aperson skilled in the art, at temperatures greater than the glasstransition point and less than their melting point, making it possibleto obtain a finished product.

Yet another subject matter of the invention is a finished productobtained by a process of thermoforming the abovementioned article to thedefinitive form, during which the matrix yarn and/or matrix fibersis/are at least partially melted in order to impregnate the reinforcingyarn and/or reinforcing fibers.

Generally, the thermoforming processes used employ low pressures (lessthan 20 bar), temperatures of less than 270° C. and short times (lessthan 5 minutes).

Other details and advantages of the invention will become more clearlyapparent in the light of the examples given below, solely by way ofindication and by way of illustration.

Matrix used: star polyamide-6, obtained by copolymerization fromcaprolactam in the presence of 0.5 mol % of2,2,6,6-tetra(β-carboxyethyl)cyclohexanone according to a processdisclosed in the document FR 2 743 077, comprising approximately 80% ofstar macromolecular chains and 20% of linear macromolecular chains, witha melt flow index, measured at 275° C. under 100 g, of 55 g/10 minutes.

EXAMPLE 1 Semi-Finished Panel Produced from Star Polyamide-6 andReinforcing Yarns

A series of tests was carried out starting from a multifilament yarn ofstar polyamide-6 exhibiting a count per strand of between 3 and 8 dTexand a tenacity in the region of 15-20 cN/Tex. Such a multifilament iscombined, during a multiaxial weaving operation, with a continuousreinforcing yarn of high performance carbon, comprising 12 000filaments, or with a reinforcing yarn of glass, exhibiting a count of600 Tex. In order to validate the high fluidity of the matrix in themolten state, multiaxial fabrics are produced from individual layers,defined as follows:

Individual Layer

-   Ply No. 1: reinforcing yarn—orientation: −45°-   Ply No. 2: reinforcing yarn—orientation: +45°-   Ply No. 3: star polyamide-6 yarn (matrix)—orientation: 90°

A laminated composite is subsequently prepared by placing severalindividual layers (between 2 and 10) of the fabric obtained in a moldexhibiting a panel form, under a heating plate press, for a period oftime of 1 to 3 minutes, under a pressure of between 1 and 20 bar and atemperature greater than the melting point of the star polyamide-6(230-260° C.). After cooling to a temperature of 50-60° C., thecomposite is removed from the mold. The level by weight of reinforcingmaterial is then between 60-70%.

The high fluidity of the star polymer makes it possible to obtain goodimpregnation of the reinforcing material by the matrix without bringingabout either the declines in mechanical properties or the problems offatigue strength observed with polymers of low molecular weight. Thebending mechanical properties are compared with those of a thermosettingcomposite obtained from the same reinforcing material and from an epoxyresin in tables Nos. 1.1 and 1.2.

TABLE 1.1 Carbon fiber composites Breaking Flexural Elongation stressmodulus at break Carbon fibers (MPa) (MPa) (%) Epoxy matrix 796.0 52 0001.72 Star PA-6 matrix 536.0 54 350 1.05

TABLE 1.2 Glass fiber composites Breaking Flexural Elongation stressmodulus at break Glass fibers (MPa) (MPa) (%) Epoxy matrix 630.0 21 0003.53 Star PA-6 matrix 580.7 21 160 3.26

The use of a reinforcing material in the form of continuous yarn makesit possible to retain superior mechanical properties in the direction ofthe unidirectional sheets of reinforcing material. The influence of thetemperature on the bending mechanical properties is given in FIG. 1.

Finally, the fact of using the matrix in the form of a yarn makespossible, in addition to an economic advantage with respect to theconventional dusting or preimpregnation solutions, easy handling andvery good control of the level of reinforcement of the final compositematerial.

A summarization of the mechanical properties obtained is given in tableNo. 3.

TABLE 3 Summarization of the mechanical properties obtained Star StarPA-6/carbon PA-6/glass multiaxial multiaxial Units Standard fabricfabric Level of % 59 65 impregnation (w/w) Density 1.4 1.8 Simpletension Tensile strength MPa ISO 527 1090 545 Young's modulus GPa ISO527 64 21.3 Elongation % ISO 527 1.7 2.76 3-Point bending Breakingstress MPa ISO 14125 536 580 Flexural modulus GPa ISO 14125 54.3 21.1Compression Breaking stress MPa ISO 604 210 195

EXAMPLE 2 Composite Braids

In order to confirm the advantage of the invention for composites with acircular cross section, braids were produced from different starpolyamide-6 yarns and from reinforcing materials chosen from those knownto a person skilled in the art, such as carbon or glass yarns.

To this end, a mixture was produced during braiding by inserting, on thebraiding machine, reinforcing yarns and polyamide yarns. The braid thusobtained is subsequently placed in a hollow mold, the braid beingmaintained by an internal bladder expanded after closing the mold.Optimum impregnation was thus obtained by virtue of the high fluidity ofthe star polyamide in the molten state, despite low processing pressures[1-5 bar]. The temperatures employed varying between 230° C. and 260°C., the impregnation time is less than 30 seconds. The composite issubsequently removed from the mold after having been cooled to below thecrystallization point of the matrix. It then exhibits a level ofreinforcing material by weight which can vary from 65 to 75%. Thesurface condition of the component is improved by virtue of the fluidityof the polymer.

1. A precursor article of a composite material comprising a polymericmatrix and at least one reinforcing yarn and/or reinforcing fibers, saidarticle comprising at least one reinforcing yarn and/or reinforcingfibers and at least one polymeric matrix yarn and/or polymeric matrixfibers, wherein: said reinforcing yarn and/or said reinforcing fibersare made of reinforcing material and optionally comprise a part made ofthermoplastic polyamide, said polymeric matrix yarn and/or saidpolymeric matrix fibers are made of thermoplastic polyamide, andwherein: said thermoplastic polyamide of said reinforcing yarn and/or ofsaid reinforcing fibers and/or of said polymeric matrix yarn and/or ofsaid polymeric matrix fibers comprises at least one polyamide possessinga star structure comprising: star macromolecular chains comprising oneor more cores and at least three polyamide branches or three polyamidesegments bonded to a core, and optionally, linear polyamidemacromolecular chains.
 2. The article as claimed in claim 1, wherein theratio by weight of the star macromolecular chains to the sum of the starmacromolecular chains and linear chains in the polyamide possessing astar structure is between 0.1 and
 1. 3. The article as claimed in claim1, wherein said polyamide possessing a star structure is obtained bycopolymerization of a mixture of monomers comprising at least: a)monomers of following general formula (I):

b) monomers of following general formulae (IIa) and (IIb):

c) and, optionally, monomers of following general formula (III):Z-R₃-Z  (III)  wherein: R₁ is an aliphatic or aromatic, cyclic orlinear, hydrocarbonaceous radical comprising at least 2 carbon atomswhich can comprise heteroatoms, A is a covalent bond or an aliphatichydrocarbonaceous radical which can comprise heteroatoms and whichcomprises from 1 to 20 carbon atoms, Z represents a primary aminefunctional group or a carboxylic acid functional group, and Y is aprimary amine functional group when X represents a carboxylic acidfunctional group, or Y is a carboxylic acid functional group when Xrepresents a primary amine functional group, R₂ and R₃, which areidentical or different, represent substituted or unsubstituted,aromatic, cycloaliphatic or aliphatic hydrocarbonaceous radicalscomprising from 2 to 20 carbon atoms which can comprise heteroatoms, andm represents an integer between 3 and
 8. 4. The article as claimed inclaim 1, wherein said polyamide possessing a star structure is obtainedby extrusion of a polyamide obtained by polymerization of lactams and/oramino acids with the compound of formula (I).
 5. The article as claimedin claim 3, wherein A represents a methylene, polymethylene orpolyoxyalkylene radical.
 6. The article as claimed in claim 3, whereinthe compound of formula (I) is2,2,6,6-tetra(β-carboxyethyl)cyclohexanone, trimesic acid,2,4,6-tri(aminocaproic acid)-1,3,5-triazine or4-aminoethyl-1,8-octanediamine.
 7. The article as claimed in claim 1,wherein the polyamide possessing a star structure has a number-averagemolecular mass of at least equal to 15
 000. 8. The article as claimed inclaim 1, wherein the matrix yarn and/or matrix fibers are obtained froma blend of the polyamide possessing a star structure and of a linearpolyamide.
 9. The article as claimed in claim 1, further comprising atleast one matrix yarn and/or matrix fibers made of linear polyamide. 10.The article as claimed in claim 8, wherein the linear polyamide is analiphatic and/or semicrystalline polyamide or copolyamide is PA-4,6,PA-6, PA-6,6, PA-6,9, PA-6,10, PA-6,12, PA-6,36, PA-11, PA-12, asemicrystalline semiaromatic polyamide or a semicrystalline semiaromaticcopolyamide chosen from the group consisting of polyphthalamides, andthe blends of these polymers and of their copolymers.
 11. The article asclaimed in claim 1, wherein the polyamide possessing a star structurehas a ratio by weight in the matrix yarn and/or matrix fibers of between0.4 and
 1. 12. The article as claimed in claim 11, wherein the ratio byweight is at least equal to 0.6.
 13. The article as claimed in claim 1,wherein the matrix yarn and/or matrix fibers further comprise additives,selected from the group consisting of flame-retardants, plasticizers,heat and light stabilizers, waxes, pigments, nucleating agents,antioxidants, and impact-strength modifiers.
 14. The article as claimedin claim 1, wherein the reinforcing yarn and/or reinforcing fibers arecarbon, glass, aramid or polyimide yarns and/or fibers.
 15. The articleas claimed in claim 1, wherein the reinforcing yarn and/or reinforcingfibers are sisal, hemp or flax yarns and/or fibers.
 16. The article asclaimed in claim 1, further comprising a powdered material which is thematrix precursor.
 17. The article as claimed in claim 16, wherein saidpowdered material is a polyamide.
 18. The article as claimed in claim 1,being in the form of continuous or cut yams, slivers, mats, braids,woven fabrics, knitted fabrics, sheets, multiaxials or nonwovens.
 19. Acomposite material, obtained from an article as claimed in claim 1, byat least partial melting of the matrix yarn and/or matrix fibers of saidarticle.
 20. The composite material as claimed in claim 19, whereinexhibiting a level of reinforcing material by weight of between 25 and80%.
 21. A semi-finished product, obtained by the process comprising thesteps of thermoforming or of calendering the article as claimed in claim1, during which the matrix yarn and/or matrix fibers is/are at leastpartially melted in order to impregnate the reinforcing yarn and/orreinforcing fibers.
 22. The semi-finished product as claimed in claim21, being in the form of panels or of tapes.
 23. A finished product,obtained by the process comprising the step of thermoforming the articleas claimed in claim 1 to a final form, wherein the matrix yarn and/ormatrix fibers is/are at least partially melted in order to impregnatethe reinforcing yarn and/or reinforcing fibers.